This invention relates to a skewing detection mechanism for a printer of the type wherein an image is formed onto a continuous recording form, for detecting transport malfunctions, such as skewing, meandering, tearing-up and the like of the continuous recording form.
Conventionally, there is known an image recording apparatus utilizing a so-called electrophotographic system in which a surface of a photoconductive drum, for example, is exposed to light to form a latent image on the drum surface, toner is then applied to the latent image to develop the same, and the developed image is transferred onto a recording medium and is fixed by a fixing unit. Such image recording apparatus is chiefly employed in a copying machine. In general, cut sheets are used as the recording medium.
In recent years, the image recording apparatus utilizing the electrophotographic system has also been employed in a printer or the like which prints the output from a computer. In such image recording apparatus, it has been desired to use, as the recording medium, a continuous recording form identical with that used in a conventional line-printer.
The continuous recording form is a folded continuous form (hereinafter referred to simply as "continuous form") called a fan-folded form which has formed therein sprocket holes. Perforations are provided at each of the folded sections to enable sheet sections to be easily severed from each other.
As a method of fixing a toner image onto the recording medium in the above electrophotographic system, there are various systems including a system due to heat fusion, a system due to solvent, a system due to pressure, and the like. The system due to heat fusion has generally been utilized, because the heat-fusion system is considered most effect in terms of safety and reliability. The heat-fusion system includes various specific ways. Among the heat-fusion ways, a so-called heat-roll fixture is most often employed, in which a recording medium having carried thereon an unfixed toner image is abutted and urged against a heated roller or a heat roller so that the toner is heated and fused due to heat applied by the heat roller.
In the heat-roll fixture, a high temperature heat roller and a backup roller are arranged in parallel relation to each other to form a fixing roll pair. The recording medium having carried thereon unfixed toner image is clamped between the rollers of the fixing roll pair, so that the recording medium is urged against the heat roller with a predetermined pressure and is heated by the heat roller. Usually, it is designed so that the recording medium is driven to travel by rotation of the fixing roll pair.
The above heat-roll fixture is also frequently employed in electrophotographic type printers utilizing a continuous form as the recording medium.
When the above heat-roll fixture is applied to the printer of the electrophotographic type printer employing a continuous form recording medium, the following problem arises. Because the continuous form is driven to travel by the fixing roll pair, the continuous form clamped between the rollers of the fixing roll pair would skew or meander due to various factors such as poor initial biting of the continuous form into the nip between the fixing rollers, unevenness or nonuniformity in thickness of the continuous form, elongation of the continuous form due to absorption of moisture, and the like. If such skewing or meandering occurs, the biting position of the continuous form with respect to the fixing rollers consecutively varies laterally, so that the side edge of the continuous form finally reaches the lateral end of the nip between the fixing rollers. This causes creases in the continuous form, resulting in defective fixing of the image and in a transport malfunction called jamming of the continuous form.
Further, the above-described defective fixing and transport malfunction occur also in a general or usual traveling drive mechanism for the continuous form. Specifically, the traveling drive mechanism comprises a pair of endless belts which are arranged respectively at opposite side edges of the continuous form. Each of the endless belts is provided with a plurality of projections which are engageable with sprocket holes formed in a corresponding one of the opposite side edges of the continuous form. The endless belts are driven to transport the continuous form. In this traveling drive mechanism, defective fixing and transport malfunctions occur due to a difference between the speed of the continuous form transported by the endless belts and the effect of traveling of the continuous form driven by the fixing rollers.
The above skewing or meandering of the continuous form does not occur in the case of cut sheets, because the cut sheets are limited in length, i.e., are relatively short in length.